M11, popularly known as the Wild Duck Cluster, (also called NGC 6705, Collinder 391, C 1848-063, Melotte 213) is a rich, compact open cluster located in the southern constellation Scutum at right ascension 18h 51.1m, declination -06°16’. It has an apparent magnitude of 6.3 and lies at a distance of 6,200 light-years from Earth. It covers an area of 14 arc minutes of apparent sky and is believed to be about 220 million years old. M11 is receding from us at 22 km/s. It is the most distant open cluster listed in the Messier catalogue that is visible to the naked eye. The cluster is metal-rich with an iron abundance of [Fe/H] = 0.17±0.04. Despite its youth, it shows an enhancement of Alpha process elements. Possibly this is due to an enhancement of its birth molecular cloud by a nearby Type II supernova explosion. The cluster contains about 2,900 stars, which makes it one of the most populated open clusters known. It is also one of the most compact clusters. The brighter members of the cluster form a V-shaped triangle that could be said to resemble a flock of ducks. About 500 of the stars in M11 are brighter than magnitude 14. The brightest, hottest main sequence stars in the cluster have the spectral classification of B8, giving an estimated age of 220 million years for M11. The cluster also contains a significant number of red and yellow giants. The stars in the cluster are only loosely bound to one another and M11 will disperse in a few million years, as its members are ejected one by one as a result of the cluster being affected by gravity from other celestial objects in the vicinity. The brightest star in M11, designated HD 174512 (HIP 92507), is a white bright giant with an apparent magnitude of 8.47. The star has the stellar classification A0 II/III and is part of a multiple star system. Messier 11 contains 82 variable stars, many of which are pulsating variables and eclipsing binary stars.
Messier 11 was discovered by the German astronomer Gottfried Kirch, director of the Berlin Observatory, in 1681. English astronomer William Derham was the first to resolve the cluster into stars around 1733. Charles Messier added the cluster to his catalogue on May 30, 1764. Swiss-American astronomer Robert Julius Trumpler classified M11 as Trumpler II.2.r, which means that the cluster is detached with little central concentration (II), has a moderate brightness range (2), and is richly populated (r). More recently, M11 has also been classified as Trumpler I.2.r, a detached cluster with strong central concentration.
The Wild Duck Cluster lies close to the northern part of the Scutum Cloud, a large star cloud that marks one of the brightest sections of the Milky Way. Messier 11 lies about a degree to the northwest of R Scuti, one of the first variable stars known. R Scuti was discovered by the English astronomer Edward Pigott in 1795 and is the first known RV Tauri type variable. The star’s brightness usually varies between magnitude 4.8 and 6, and occasionally between magnitude 4.5 and 8.2. The cluster can be seen above the Teapot asterism in Sagittarius constellation. It can be located by first finding the bright Altair in the constellation Aquila, then following the line to the southwest to Denebokab, Delta Aquilae, and finally to Lambda Aquilae. Altair, the 12th brightest star in the night sky, is the second brightest star in the Summer Triangle, a prominent asterism high overhead in the summer, formed by Altair and the bright stars Vega in the constellation Lyra and Deneb in Cygnus. Altair marks the southern corner of the triangle. To the southwest of Lambda Aquilae lies a fainter star, 12 Aquilae, and M11 is located just over 2 degrees west of the star. The best time of year to observe the cluster is from June to September when it rises high in the night sky. Credits: Messier Objects, NASA, Universe Today, Wikipedia.